A hydrostatic transmission employed in a working vehicle is generally composed of a hydraulic pump of a variable displacement type and a hydraulic motor of a fixed displacement type for the purpose of avoiding complication not only in structure but in speed-changing operations. A four wheel-drive transmission for working vehicles is usually adapted to drive a pair of primary vehicle-drive wheels (rear wheels or front wheels) constantly while permitting a selective drive of a pair of auxiliary vehicle-drive wheels (front wheels or rear wheels).
A four wheel-drive transmission, including a hydrostatic transmission, of the type set forth above which is considered to be of the simplest structure is such, as disclosed in JP, U No. 3-73751, in that a hydrostatic transmission adapted to drive rear wheels constantly through a rear wheel-drive assembly is provided and in that there is arranged at a front side of the hydrostatic transmission a front wheel-drive assembly which comprises a front wheel-drive shaft for taking-off front wheel-driving power and a front wheel-driving clutch for selectively connecting the drive shaft to a front end of the output shaft of the hydrostatic transmission. However, this structure cannot be employed in a drive transmission in which the rear wheel-drive assembly includes a speed change mechanism for setting a low-speed range, required when the vehicle is used in working operations, and a high-speed range required when the vehicle is traveled on roads. This is because, while the peripheral speed of rear wheels is changed in response to a shifting operation of the speed change mechanism, the peripheral speed of front wheels driven to rotate through the front wheel-drive shaft remains unchanged, so that a condition may be caused where there is much difference in peripheral speed between front wheels and rear wheels.
Thus, in a transmission in which the rear wheel-drive assembly includes a speed change mechanism for changing or setting vehicle speed ranges, a front wheel-drive shaft is provided which is connected to a transmission shaft of the driven side of the speed change mechanism as shown, for example, in U.S. Pat. No. 4,579,183. However, such a structure will cause problems that transmission casing for housing the speed change mechanism is enlarged in size and that the hydrostatic transmission arranged at a front side of the transmission casing provides a restriction to the arrangement of a transmission shaft for transmitting power from the front wheel-drive shaft toward the front wheels so that the transmission shaft cannot be arranged at an appropriate level or in a laterally central location.
Accordingly, a primary object of the present invention is to provide a novel vehicle drive transmission in which vehicle speed ranges can be set by a hydrostatic transmission itself so as to eliminate a speed change mechanism of the type set forth above and to thereby permit an easy employment of a four wheel-drive structure.
An attendant object of the invention is to provide a transmission in which speed-changing operations are simplified while reducing the cost of the transmission largely.
In view of the fact that a working vehicle such as a tractor is used for various working operations and in view of the fact that such a vehicle is generally traveled at a relatively high speed when it is traveled on a road, it is another object of the present invention to provide a novel transmission which provides plural drive modes for the vehicle.
An attendant object of the invention is to provide a four wheel-drive structure which permits an easy conversion to a two wheel-drive structure so as to contribute to reducing the cost of these drive structures.
Another attendant object is to provide a four wheel-drive transmission which contributes to reducing fuel cost while stabilizing the traveling condition of vehicle.